1. Technical Field of the Invention
The present invention relates to a signal processor for a gas sensor.
2. Related Art
This type of gas sensor is in practical use as an A/F (air-fuel ratio) sensor, for example, provided at an exhaust passage of an internal combustion engine. The air-fuel ratio of an air-fuel mixture is detected based on the output of the sensor. In an air-fuel ratio control system for an internal combustion engine, air-fuel ratio feedback control is performed in order to have the actual air-fuel ratio that has been detected by the A/F sensor matched with a target air-fuel ratio at every instant.
It is considered that, in the exhaust passage, variation is caused in the exhaust pressure (exhaust pulsation), which causes variation in the output of the A/F sensor induced by the exhaust pulsation. To take measures against the variation in such sensor output induced by exhaust pulsation, Japanese Patent Application Laid-Open Publication No. 2004-150379, for example, suggests a technique with which a moving average process is performed in respect to a plurality of outputs. Performing the moving average process can moderate sensor outputs, whereby an A/F detection value that has been suppressed with the exhaust pulsation can be obtained.
A moving average process performed in respect to a plurality of outputs as mentioned above may reduce the variation in the sensor output, which variation is induced by exhaust pulsation. However, there has been a concern that the moving average process may deteriorate the responsiveness of the sensor output to actual changes of the gas atmosphere (gas concentration change). Specifically, although output occurs in an A/F sensor in response to the actual gas concentration change, the execution of the moving average process by a microcomputer or the like may disable detection of the intrinsic sensor output, or may increase response delay for the intrinsic sensor output.
Recent study, in particular, has put a focus on detecting the air-fuel ratio of each cylinder in a multi-cylinder internal combustion engine and on effecting air-fuel ratio control for each cylinder based on the result of the detection of the air-fuel ratio of the cylinder, in order to further improve the exhaust emissions of the internal combustion engine. Such a cylinder-specific air-fuel ratio is detected based on the output of an A/F sensor provided at an exhaust concentrated portion in the multi-cylinder internal combustion engine. Disadvantageously, however the moving average process performed in respect to the outputs of the sensor as mentioned above may disable highly-responsive detection of the cylinder-specific air-fuel ratio.